This investigation will lead to a better understanding of the regulation of protein synthesis and degradation under normal conditions. The study of how it is altered in an attempt by the organism to meet the nutritional crisis brought about the lack of protein or calories is important and imperative. The rates of protein synthesis will be measured by the constant infusion of (14C) tyrosine. The fractional rates of protein breakdown will be estimated in muscle from the decay in (1-14C) glutamate radioactivity and in liver from the decay in amino acids labeled by (14C) carbonate. The mechanisms of regulation of protein synthesis and degradation will be sought from the study of protein turnover of total tissue proteins and, in particular, of single, isolated proteins wherever possible in rats during nutritional deprivation, in rats of different age, and in animals of different size, viz, rats and swine.